Kamal Foroughi, right, with his son Kamran, pictured at Oriel College, Oxford, in 1998.

A 76-year-old British national has been held in an Iranian jail for more than four years and convicted of spying, his family has revealed, as they seek to draw attention to the plight of a man they describe as one of the “oldest and loneliest prisoners in Iran”.

Kamal Foroughi, a businessman who was working in Tehran as a consultant for the Malaysian national oil and gas company Petronas, was arrested in May 2011 when plainclothes officers picked him up from his flat in the Iranian capital. They did not show a warrant for his arrest, according to his family.

Foroughi was held at the notorious Evin prison and eventually sentenced in 2013 to a total of eight years’ imprisonment, which he is still serving in Evin. The news about his arrest has only just come to light after his family decided to break their silence.

“My dad’s detention has been a total nightmare for all the family,” Foroughi’s son, Kamran, told the Guardian. “My dad has always strenuously maintained his innocence and we believe him. We are not aware of any evidence that justifies the espionage charge. My daughters have lived half their lives without seeing Grandpa and keep asking when he is coming home. All I can do is give them a hug.”

Foroughi’s consultancy role at Petronas included arranging and participating in meetings with senior Iranian oil and gas officials and facilitating scholarships for Iranian students on a government bursary to study in Malaysia.

It is not clear on what evidence Iran convicted him on spying charges but Foroughi holds both British and Iranian citizenships. Iran’s intelligence apparatus view dual nationals with deep suspicion and have arrested a number of them in recent years, including the Iranian-American journalist Jason Rezaian, who is currently facing similar charges.

In 2013, an upper court sentenced Foroughi to seven years in jail for espionage and an extra year for possessing alcohol at home. Consuming alcohol is a crime under Iranian law. His family said they have repeatedly contacted the authorities at Iran’s judiciary both directly and through his lawyer but have not had much clarity on the exact reasons for his spying charges.

His case was presided over by the hardline Iranian judge Abolghassem Salavati, who is notorious for handing down heavy sentences against those held in jail on political grounds.

Kamran Foroughi said he feared that his father’s friendship with a former British ambassador in Tehran might have caused him trouble. But Iran has not publicly commented on his detention and he has had little access to his laywer.

The British Foreign Office said it has been aware of the case since 2013 and has provided assistance to the family but it has not been able to provide consular access to Foroughi because Iran does not recognise dual nationality and treats him as solely Iranian.

A Foreign Office spokesman said: “We are very concerned for Mr Foroughi’s health and have raised this with the Iranian authorities on repeated occasions, urging them to provide regular medical assistance and access to a lawyer.

“The foreign secretary raised this case with President Rouhani during his visit to Tehran in August, and more recently with Foreign Minister Zarif at the UN, just two weeks ago. We will continue to raise it with the Iranian government at every opportunity, seeking Mr Foroughi’s release on medical grounds.”

The Guardian understands that the prime minister, David Cameron, has also raised the issue at the highest level with the Iranian authorities.

Foroughi’s son said he fears his father may suffer from a number of medical conditions and could need urgent medical care – especially access to cancer specialists.

“We are very worried about his health and are concerned he may die in prison,” Kamran Foroughi said. “He has been waiting for 14 months for a full medical checkup authorised by the Iranian authorities to consider clemency, and we have received no information on the brief medical checks that have been made.”

He said with the thawing of the bilateral relations between Tehran and London and the reopening of the embassies in August, he hopes the Iranian authorities would show compassion and release his father soon.

“My dad is 76 years old and has not seen any of his family for over four years – his wife, two children and two granddaughters all live in the UK,” he said.

“For over three years we had no contact with him. During the last year we have been allowed regular short calls with him, for which we are very grateful. However, no written correspondence, photos or videos are allowed. Family or friends cannot visit him. He is one of the oldest and loneliest prisoners in Iran.”